U.S. Pat. No. 6,329,487 to Abel et al discloses a process for preparing novel silazanes and polysilazanes by ammonolysis of a chlorosilane, for example, or a mixture of chlorosilanes comprising the addition of these chlorosilanes to liquid anhydrous ammonia. There are many advantages to such a process over other processes in preparing silazanes and polysilazanes, as detailed by Abel et al.
A disadvantage of this process, however, resides in the handling and/or disposal of the solution of ammonium halide in liquid anhydrous ammonia, which results as a by-product of the reaction. On the one hand, the ammonium halide solution can be dealt with by simply evaporating the liquid anhydrous ammonia from the solution to leave dry, ammonium halide salt. The ammonia gas which results can be recovered and recondensed back to liquid form and reused in the process.
While the above ammonia recovery process is attractive from an economical standpoint, it was found that a great deal of the ammonia originally introduced in the process is lost as ammonium halide salt. It is believed that the problem with simply allowing the ammonia to evaporate is due, at least in part, to the problem associated with the ammonium halide salt which precipitates out of solution in the process. To wit, as the ammonia evaporates from the solution of solubilized and ionized ammonium halide salt in liquid ammonia the concentration of the ammonium halide salt increases. As the concentration of the salt increases during evaporation the adsorption of residual ammonia onto the precipitated solid salt increases, resulting in physical entrapment of residual liquid anhydrous ammonia within the solid ammonium halide salt. This encumbers evaporation and recovery of the ammonia from the solution of ammonium halide salt in liquid anhydrous ammonia. Additionally, the problem of waste disposal of the ammonium halide salt remains, so that environmental concerns are not addressed as fully as one would desire.
Accordingly, there is a need for a more efficient process for the recovery of ammonia from an ammonolysis reaction in the production of silazanes and polysilazanes, and one that results in a less hazardous, more readily disposable salt residue.